Na2CaSn2Ge3O12: A Novel Host Lattice for Sm3+-Doped Long

Article pubs.acs.org/IC

Na2CaSn2Ge3O12: A Novel Host Lattice for Sm3+-Doped LongPersistent Phosphorescence Materials Emitting Reddish Orange Light Jiao Xu, Zhenghua Ju, Xiuping Gao, Yanqing An, Xiaoliang Tang, and Weisheng Liu* Key Laboratory of Nonferrous Metals Chemistry, Resources Utilization of Gansu Province, and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China S Supporting Information *

ABSTRACT: A novel host lattice disodium calcium ditin(IV) trigermanium oxide Na2CaSn2Ge3 O12 was utilized for synthesizing long-persistent phosphorescence materials for the first time. Reddish orange long-persistent phosphorescence was observed in Na2CaSn2Ge3O12:Sm3+ phosphors with persistence time more than 4.8 h. The phosphors were synthesized by a conventional solid-state reaction pathway in air atmosphere. A predominant cubic phase of Na2CaSn2Ge3O12 was observed in all XRD patterns. Photoluminescence measurements indicated that the emission spectrum was composed of the peaks located at 566 (the strongest), 605, 664, and 724 nm. The results of the decay curves in terms of a biexponential model suggest that different defects appear in the crystal lattice. The defects acting as traps were investigated by thermoluminescence, which demonstrated that doping Sm3+ ions into the Na2CaSn2Ge3O12 host has made the trap types abundant. Furthermore, the origin of the longpersistent phosphorescence has also been discussed. On the basis of the above results, Sm3+-doped Na2CaSn2Ge3O12 phosphors are considered to have potential practical applications.

■

developed a new red LPP phosphor CaTiO3:Pr3+. The afterglow time of the phosphor is too short, although it is approximately ideal red.12 Murazaki et al. reported a red LPP phosphor Y2O2S:Eu3+,Mg2+,Ti4+, which shows a longer red afterglow time. The S as a sulfurization agent in the process is harmful to environment.13 In recent years, red LPP of MO:Eu3+ (M = Ca, Sr, and Ba)14 and M2SnO4:Sm3+ (M = Ca, Sr, and Ba)15−17 were reported. But, so far, no perfect red or reddish orange LPP phosphor has been obtained in practical application. Theoretically, the occurrence of LPP relies on two kinds of active centers which exist in phosphors, emitters, and traps. Emitters can emit radiation after being excited. Traps with suitable depth are able to store excitation energy and immobilize it for an appropriately long time, and then release and transfer it to the emitters by degrees as a result of external thermal stimulations.18 The emission wavelength of LPP mainly depends on the emitter, while the afterglow intensity and duration time are depending on the property of the trap. Therefore, a proper emitter which is capable of emitting reddish orange light and a suitable host which is able to create appropriate traps and generate persistent luminescence are required in designing reddish orange LPP phosphors. Among

INTRODUCTION Long-persistent phosphorescence (LPP) materials can store the absorbed light energy and release it in the form of luminescence after stoppage of the excitation.1−3 Recently, great attention has been attracted to design and construct new excellent LPP phosphors which are potentially applied in the fields such as safety indication, detection of high-energy rays, road signs, automobile instruments, and so on.3−6 In theory, different color-emitting LPPs can be obtained through mixing the three primary colors (blue, green, and red) emitting LPPs with similar properties (chemical and physical) at = proper proportion.6 LPP phosphors for two of the tricolor, blue7 and green,4 which possess high brightness, long persistent time, and good chemical stability, are commercially available. However, the luminosity, and/or the duration time of the LPP materials which emit in the long wavelength (orange to red) region still cannot meet the needs of commercial application.8 Furthermore, considering the biomedical research and medicine applications, the emission wavelength at the longer region from 650 nm to the infrared is more suitable for use as luminescent probes for in vivo imaging.9 Hence, there is a strong desire for exploring excellent reddish orange LPP. In the earlier times of this research, divalent europium ion (Eu2+) activated sulfides such as MS:Eu2+ (M = Ca, Sr)10,11 were used to prepare red LPP phosphors, but they are chemically unstable and sensitive to moisture. Diallo et al. © 2013 American Chemical Society

Received: May 20, 2013 Published: November 21, 2013 13875

dx.doi.org/10.1021/ic401262w | Inorg. Chem. 2013, 52, 13875−13881

Inorganic Chemistry

■

all RE3+ ions, the samarium (Sm3+) ion is famous for generating intense reddish orange emitting light which was ascribed to the typical transitions between the ground state and the excited state electron configuration of Sm3+. So Sm3+ is a versatile optically active ion for various inorganic host lattices.19−24 Owing to all the above-mentioned reasons, we want to find a perfect host which is able to create appropriate traps and generate reddish orange LPP when doped with Sm3+. Na2CaSn2Ge3O12, which is a cubic A2B3C3X12-type structure with the space group of Ia3̅d (No. 230), has shown more attraction than sulfide or oxysulfide as host owing to its high chemical stability, environmental friendliness, and simple preparation technique. The most important point is that it also can implant other ions such as RE3+ ions into the host lattice to produce phosphors emitting a variety of colors. As we all know, solid-state reaction through sintering the mixture of solid starting materials is the most popular and useful method for synthesis of polycrystalline solids in industrial applications. In this work, we have successfully prepared a series of novel Sm3+-doped Na2CaSn2Ge3O12 reddish orange LPP phosphors through a conventional solid-state method in air atmosphere. The oven temperature is 1250 °C. The results show that the Sm3+ concentration can affect the phosphorescent properties of Na2CaSn2Ge3O12:Sm3+, such as luminescence intensity and afterglow time. The effects of the doping amount of Sm3+ in Na2CaSn2Ge3O12 on the properties of luminescence and afterglow were discussed. The dynamical processes of the LPP were studied, and the formation mechanism of Na2CaSn2Ge3O12:Sm3+ LPP phosphors was also investigated.

■

Article

RESULTS AND DISCUSSION Crystal Structure. The structural purities of Na2Ca1‑xSn2Ge3O12:xSm3+ samples were studied by the powder X-ray diffraction. Figure 1 shows the XRD patterns of S0, S1, S2,

Figure 1. XRD patterns of S0, S1, S2, S3, S4, S5, S6, S7 samples and the standard data for Na2CaSn2Ge3O12 as reference.

S3, S4, S5, S6, S7 samples and the standard data for Na2CaSn2Ge3O12 as reference. The results reveal that a predominant phase of Na2CaSn2Ge3O12 (JCPDS No. 720105) is presented in all powder samples and doping Sm3+ does not make any appreciable changes in the host structure.25 Figure 2 represents the unit cell of the Na2CaSn2Ge3O12 host.

EXPERIMENTAL SECTION

Sample Preparation. Powder samples with nominal composition of Na2Ca1‑xSn2Ge3O12:xSm3+ (where x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, and 1.4 mol %, hereafter labeled as sample S0, S1, S2, S3, S4, S5, S6, and S7) are synthesized by conventional solid-state reaction. The powders of Na 2 CO 3 (99.8%), CaCO 3 (99.0%), SnO 2 (99.8%), GeO 2 (99.999%), and Sm2O3 (99.99%) acted as raw materials, and the corresponding flux of 2 mol % (NH4)2C2O4·H2O (99.5%) and 2 mol % H3BO3 (99.5%) were used in each sample. Stoichiometric molar ratios of the raw materials and flux were thoroughly homogenized using a wet grinding method in an agate mortar and pestle (ethanol was added as the dispersing liquid). Finally, the alumina crucible with the mixture was put into the muffle furnace and heated at 1250 °C for 5 h in air. After cooling, the products obtained were ground. Characterization. The crystalline structures of the phosphor powders were checked by X-ray diffraction (XRD) on a RigakuD/ MAX-2400 powder diffractometer using Cu Kα radiation (1.5405 Å) in a 2θ interval from 10° to 90°. The UV−vis diffuse reflectance spectra were obtained by a Varian Cary 100 UV−vis spectrophotometer (the calibrator is BaSO4 powder, reflection ∼100%). An Edinburgh FLS 920 combined fluorescence lifetime and steady state spectrometer with Xe 900 (450 W xenon arc lamp) as the light source was utilized to record the photoluminescence excitation (PLE) and emission (PL) spectra of powders. The decay curves were measured immediately after the samples were activated under 254 nm UV lamp for 10 min by obtaining the signal on a PR305 phosphorophotometer (Zhejiang University Sensing Instruments Co., Ltd., China). Thermoluminescent (TL) glow curves were carried out using a FJ427A TL meter (Beijing Nuclear Instrument Factory) at the temperature range between 280 and 650 K with 1 K/s calefactive rate. The amount of all samples was fixed at 0.0020 g. The samples were irradiated by 254 nm lamp for 10 min prior to the measurements. Except for the TL measurements, the rest of the measurements were performed at room temperature.

Figure 2. Schematic drawing of the crystal structure of Na2CaSn2Ge3O12.

In this unit cell, Na and Ca ions are both located at the 24c site. Sn ions are located at the 16a site. Ge ions are believed to be located at the 24d site, and O ions are located at the 96h site. The Sn site bonds to six O atoms to form an octahedron, and the Ge site is coordinated by four O atoms and forms a GeO4 tetrahedron. Structurally Na2CaSn2Ge3O12 belongs to the cubic A2B3C3X12-type structure with the space group Ia3d̅ (No. 230).26 In this structure, each SnO6 octahedron is joined to six GeO4 tetrahedrons, each GeO4 tetrahedron is joined to four SnO6 octahedrons, and the octahedrons and tetrahedrons are 13876

dx.doi.org/10.1021/ic401262w | Inorg. Chem. 2013, 52, 13875−13881

Inorganic Chemistry

Article

Figure 3. (a) UV−vis diffuse reflectance spectrum of undoped Na2CaSn2Ge3O12 (sample S0), (b) relationship (K*hv)1/2 = f(h*v).

4a) was recorded by monitoring at 566 nm. It can be seen clearly that the PLE spectrum shows a strong broad band from 230 to 300 nm (maximal value positioned around 255 nm) due to host absorption and coincides precisely with the band labeled “A” appearing in the diffuse reflectance spectrum (Figure 3a). The PLE spectrum of S5 exhibiting a strong excitation band of the host lattice in the ultraviolet (230−300 nm) region confirms the energy transfer from the host to the Sm3+ ions. Because Sm3+ ions can acquire energy from the host, the 4f → 4f intraconfigurational transitions of Sm3+ will appear when excited by the UV light source. So, except the band at about 230−300 nm, other bands can be observed in the 320− 500 nm range with the peaks centered at 365, 404, 434, 454, 468, 473, and 482 nm corresponding to the transitions from the ground level 6H5/2 to the excited levels 4H7/2, 6P7/2, 4F7/2, 6P5/2, 4 G9/2, 4I11/2, and 4I13/2 of the Sm3+, respectively.22,23 When S5 phosphor was excited under 255 nm UV lamp, the emission spectrum (Figure 4b) was formed by four sets of lines in the reddish orange spectral region (550−720 nm) which are ascribed to the de-excitation from 4G5/2 of Sm3+ to its lower multiplets of 6HJ (J = 5/2, 7/2, 9/2, 11/2), and the 4G5/2 → 6H5/2 emission peak at 566 nm is the strongest one. The phenomenon of one transition featuring two emission peaks is considered to be caused by crystal field splitting. According to the selection rules,30 the 4G5/2 → 6H5/2 transition mainly arising from magnetic dipole, the electric dipole mechanism played a key role in the transitions of 4G5/2 → 6HJ (J = 7/2, 9/2, 11 /2). To observe the effect of the doping concentration of Sm3+ on the luminescence intensity, we prepared a series of Na2CaSn2Ge3O12:Sm3+ phosphors with different concentrations of Sm 3+ . Figure 5 shows the PL spectra of Na2Ca1−xSn2Ge3O12:xSm3+ with different x values, and two points are worth noticing. First, the peak position and the shape of the emission spectrum are identical in different samples; second, the intensity of the emission changes with increasing concentrations of Sm3+. The dependence of 566 nm emission intensity on Sm3+ doping concentration is presented in the inset of Figure 5. It is evident that the variation of emission intensities appears just like a part of a parabola and reaches a maximum at the values of x = 1.0%. The concentration quenching may been induced by cross relaxation processes in close Sm3+−Sm3+. When the Sm3+ concentration increases, the possibility of energy transfer between Sm3+ ions increases.31 According to the report of Blasse,32 we can roughly estimate the critical distance (the average shortest distance between the

connected by corner-sharing, which results in the formation of the A2B3C3X12-type skeleton. The Na2CaSn2Ge3O12 structure contains three-dimensional network of corner-sharing SnO6 octahedrons and GeO4 tetrahedrons; the channels in the threedimensional network are occupied by Na cations or Ca cations. As mentioned above, the ionic positions of Na+ and Ca2+ are the same. Sm3+ (RSm3+ = 0.90 Å) can substitute for the Ca2+ (RCa2+ = 0.94 Å) or Na+ (RNa+ = 0.96 Å) in the tetracoordinate state according to the ionic radius and valence states. UV−Vis Diffuse Reflectance Spectra. Figure 3a shows the UV−vis diffuse reflectance spectrum of S0 phosphor. Two band edges around 255 nm (A) and 300 nm (B) can be observed in the sample. They were known as the charge transfer transitions in the stannate15 and the germanate host,27,28 respectively. Relationship (K*hv)1/2 = f(h*v) is shown in Figure 3b. The Kubelka−Munk transformation of diffuse reflectance data of S0 sample is performed by the following function K=

(1 − R )2 2R

(1)

where K is reflectance transformed according to Kubelka− Munk, h is Planck constant, v is the light frequency, and R is reflectance (%). The band gap (Eg) of Na2CaSn2Ge3O12, the host lattice, is estimated to be 3.43 eV.29 Luminescence Properties of Sm3+-Doped Na2CaSn2Ge3O12. The PLE and PL spectra of a selected sample S5 are depicted in Figure 4. The PLE spectrum (Figure

Figure 4. PLE (λem = 566 nm, a) and PL (λex = 255 nm, b) spectra of S5 phosphor. 13877

dx.doi.org/10.1021/ic401262w | Inorg. Chem. 2013, 52, 13875−13881

Inorganic Chemistry

Article

⎛ −t ⎞ ⎛ −t ⎞ I(t) = I0 + A1 exp⎜ ⎟ + A 2 exp⎜ ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

(3)

Here, I(t) and I0 are the phosphorescence intensities at time t and 0, A1 and A2 are constants, t is time, and τ1 and τ2 are the decay times. The fitting parameters also were listed in Table 1. Table 1. Constants (A) and Decay Times (τ) of Na2CaSn2Ge3O12:Sm3+ as a Function of Concentrations of Sm3+ sample S1 S2 S3 S4 S5 S6 S7

Figure 5. PL spectra of S1, S2, S3, S4, S5, S6, S7 samples. The inset picture shows the intensity of 566 nm emission as a function of Sm3+ concentration in Na2Ca1−xSn2Ge3O12:xSm3+ samples (λex = 255 nm).

nearest activator ions) of energy transfer (Rc), and calculated as follows:33−35 ⎛ 3V ⎞1/3 R c ≈ 2⎜ ⎟ ⎝ 4πxcN ⎠

A1

A2

τ1 (s)

τ2 (s)

144.1 186.7 230.8 269.1 311.0 290.2 257.0

41.7 47.9 62.7 87.1 95.5 92.9 86.1

4.5 4.6 5.0 5.8 6.3 6.0 5.5

47.8 49.3 50.1 54.3 59.5 57.4 52.6

These parameters further demonstrated that the decay processes of S5 phosphor possessed a biexponential decay character. Initially, intensity of the afterglow decreases rapidly and then decays very slowly.23,36 Owning to the slow decay process, the afterglow emission of the S5 phosphor lasts for more than 4.8 h after removal of the 254 nm UV light. The decay curves (as shown in Figure 7) of all samples are in accord

(2)

Here V is the unit cell volume, xc is the critical concentration, and N is the number of ions in a unit cell. In Na2CaSn2Ge3O12, V = 1866.6 Å3, N = 4, and the critical concentration (xc) is about 0.01 in our system. The Rc value is about 44.6 Å obtained by using eq 2. Usually, the color of the phosphor can be shown by color coordinates. The chromaticity coordinates were calculated on the basis of the measured PL spectra. In this work, the chromaticity coordinates were obtained by calculation from the spectrum in Figure 4b. The chromaticity coordinates (x, y) of S5 phosphor are (0.55, 0.40), which locates in the typical reddish orange region. LPP Properties of Na2CaSn2Ge3O12:Sm3+ Phosphors. A novel reddish orange LPP phosphor family Na2Ca1‑xSn2Ge3O12:xSm3+ was discovered in our present work. The decay and fitting curves of S5 phosphor are presented in Figure 6. It is clearly exhibited that the decay process consists of a fast decay process and a slow decay part which are well-fitted into a biexponential function as follows:

Figure 7. Afterglow decay curves of Na2Ca1‑xSn2Ge3O12:xSm3+ (x = 0.2, 0.6, 1.0, 1.4 mol %) phosphors after ceasing the 254 nm UV excitation source.

with eq 3. The obtained parameters also are listed in Table 1. It is clearly observed from Table 1 that A and τ are enhanced by the addition of Sm3+. The maximum enhancement occurs in 1.0 mol % Sm3+ added sample. From the above results, we infer that the Sm3+ doping concentration in the Na2CaSn2Ge3O12 matrix can influence the afterglow properties of Na2CaSn2Ge3O12:Sm3+ samples. The biexponential decay model of Na2CaSn2Ge3O12:Sm3+ phosphor was fully consistent with the behavior of many other ions doped LPP phosphors.37,38 The effect of dopant concentration on the afterglow properties was evaluated by varying the concentration of Sm3+ in Na2CaSn2Ge3O12 samples. The decay curves of Na2Ca1‑xSn2Ge3O12:xSm3+ (x = 0.2, 0.6, 1.0, 1.4 mol %) are depicted in Figure 7. For all the curves, the decay rate of the afterglow intensity is very high at first and then become low. The Sm3+ quenching concentration begins with x = 1.0 mol %.

Figure 6. Afterglow decay curve (●) of S5 phosphor (line shows the fitting result) after being activated for 10 min under 254 nm UV light. 13878

dx.doi.org/10.1021/ic401262w | Inorg. Chem. 2013, 52, 13875−13881

Inorganic Chemistry

Article

Figure 8. (a) TL curves of Na2Ca1−xSn2Ge3O12:xSm3+ phosphors with different Sm3+ concentration. (b) TL curves of S5 sample, and black solid line is the measured curve. The dotted lines and green, and blue solid lines are the simulated Gaussian curves. The insert picture of part b shows the TL curves of S0 sample, and solid line is the measured curve. The dotted lines are the simulated Gaussian curves.

and 0.35 eV, respectively. The ca. 311 and 343 K TL peaks of the S5 phosphor have not noticeably altered in comparison with that of the undoped S0 matrix. However, two different peaks which locate at 318 and 348 K are present in the glow curve of the S5 phosphor elucidating that doping Sm3+ into the Na2CaSn2Ge3O12 matrix not only enhances the trapping capacity but also creates foreign trap centers. Because the ionic positions of Na+ and Ca2+ are the same and the ionic radii of Na+ (RNa+ = 0.96 Å), Ca2+ (RCa2+ = 0.94 Å), and Sm3+ (RSm3+ = 0.90 Å) ions are very close, the Sm3+ ions would chemically nonequivalently substitute the Ca2+ sites or the Na+ sites when they were introduced into the Na2CaSn2Ge3O12 host matrix. The excess positive charge which was generated by Ca2+ or Na+ being nonequivalently replaced by Sm3+ in the lattice must be compensated for in order to keep the phosphor electrically neutral. The possible approach to achieve the charge compensation of the Sm3+-doped Na2CaSn2Ge3O12 phosphor is two Sm3+ ions substituting for three Ca2+ ions which result in the formation of two positive charge defects (Sm°ca) and one negative defect (V′Ca′ ), or one Sm3+ ion substituting for three Na+ ions, which results in the formation of one positive charge defect (SmNa °°) and two negative defects (VNa ′ ).15,36 The equation about the formation of defects are expressed as follows:

The lifetime decreased markedly from the concentration of Sm3+ exceeding 1.0 mol %. The results of the effect of Sm3+ concentration on the afterglow lifetime of Na2CaSn2Ge3O12:Sm3+ phosphor are in agreement with the conclusions obtained by Figure 5. Thermoluminescence Properties. It is well-known that LPP is due to charge carriers (i.e., holes, electrons, or their pairs) which can be trapped in abundant defect (intrinsic or extrinsic) centers and then are continuously released under the action of thermostimulation.39 The traps which were made up of the defects in the host lattice play an important role in the afterglow properties in LPP phosphors. Hence, the knowledge of the traps must be studied thoroughly in order to investigate the formation of LPP. The thermoluminescence (TL) technique is a helpful tool for evaluating the density and depth of traps generated in materials under the irradiation of UV light.40,41 In order to study the parameters of the trapping centers produced under the excitation of ultraviolet light and further investigate the significant effect of different traps on the origin of the LPP of the Na2CaSn2Ge3O12:Sm3+ phosphors, the measurement of the TL glow curves of these samples was performed. The TL curves of samples S1−S7 in the temperature range 280−650 K are shown in Figure 8a. Figure 8b and the insert picture of Figure 8b represent the TL curves of the S5 and the S0 matrix phosphors, respectively. The TL curves of the S0 and S5 phosphors were deconvoluted by curve fitting technique based on the Gauss equation.42 Consequently, Chen’s method43 was used to analyze the individual deconvoluted peaks by the equation ⎛ k T2 ⎞ E = 2.52 + 10.2(μg − 0.42)⎜ B m ⎟ − 2kBTm ⎝ ω ⎠

x Sm°Ca + V″Ca ≫ SmCa + V′Ca

(5)

° x Sm°° Na + V′Na ≫ Sm Na + V Na

(6)

The IPs of Na and Ca are 47.3 and 51.0 eV, respectively.44 The trap depths associated with these cation vacancies would decrease in the order Ca2+ > Na+.45 Hence, the 311 and 343 K TL peaks are correlated with the Na+, Ca2+ vacancies, respectively. This attribution has already been confirmed by the fact that these TL peaks also appeared in the Na2CaSn2Ge3O12 host as shown in the insert picture of Figure 8b. One of the commonly used methods to get excellent performance of LPP phosphors is by introducing auxiliary ions which have different valences and ionic radii compared with the host cations into the substrate.46−48 In this work, apart from being an activator, trivalent Sm3+ ion can also play the role of aliovalent dopant to create defects. On the basis of the abovediscussed result, the 318 and 348 K TL peaks may originate from the positive defects created by Sm3+ ions replacing Ca2+ and Na+ ions. Possible Mechanism of the Na2CaSn2Ge3O12:Sm3+ Phosphors. On the basis of the above-mentioned results, a +

(4)

where E is the trapping level, kB is Boltzmann’s constant, μg is the symmetry factor, defined as μg = δ/ω, ω = τ + δ, in which ω is the total half intensity width, τ is the left half width, and δ is the right half width. Tm is the temperature corresponding to peak intensity. The TL glow curves shown in the insert picture of Figure 8b reveal that in the S0 matrix there are two different kinds of traps responsible for the two peaks in the TL curve situated at 311 and 343 K, respectively. At least four Gaussian curves were obtained in the TL curve of the S5 phosphor, the peaks of the four Gaussian curves located at around 311, 318, 343, and 348 K, respectively. The values of E calculated for TL peak 1, peak 2, peak 3, and peak 4 are about 0.26, 0.55, 0.68, 13879

2+

dx.doi.org/10.1021/ic401262w | Inorg. Chem. 2013, 52, 13875−13881

Inorganic Chemistry

Article

results indicated that different defects exist in all samples. The LPP is generated through the recombination of the thermally released holes and electrons which were trapped in the Na2CaSn2Ge3O12 matrix. Due to the Na2CaSn2Ge3O12:Sm3+ phosphors possessing numerous suitable stable traps which are able to permanently store energy at room temperature, they can serve as the storage phosphor.

possible mechanism was proposed to explain the generation of reddish orange LPP in Na2CaSn2Ge3O12:Sm3+ phosphors. So far, there is still lack of a convincing mechanism of the LPP. Nowadays the thermostimulated recombination of holes and electrons is generally accepted among all the reports about the LPP mechanism.6,36,40,49−51 Phosphorescence mechanism of Na2CaSn2Ge3O12:Sm3+ can be explained as follows: part of the defects may serve as hole capture centers, while the others may serve as electron capture centers. In our present work, Sm3+ is an activator itself as well as the trap provider. A schematic diagram of LPP mechanism for Na 2 CaSn 2 Ge 3 O 12 :Sm 3+ phosphors is displayed in Figure 9. The band gap of

■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Additional experimental details, data, tables, and figures. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Phone: +86-931-8915151. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work is financially supported by the NSFC (20931003, and 91122007), the Specialized Research Fund for the Doctoral Program of Higher Education (20110211130002), and the Fundamental Research Funds for the Central Universities (lzujbky-2012-24).

■

Figure 9. Schematic diagram that presents a possible trapping mechanism of the Na2CaSn2Ge3O12:Sm3+ phosphors. ● denotes electron trap; ○ denotes hole trap.

REFERENCES

(1) Hölsa, J. Electrochem. Soc. Interface 2009, 18, 42−45. (2) Luitel, H. N.; Watari, T.; Chand, R.; Torikai, T.; Yada, M.; Mizukami, H. Mater. Sci. Eng., B 2013, 178, 834. (3) Zhang, J. C.; Yu, M. H.; Qin, Q. S.; Zhou, H. L.; Zhou, M. J. J. Appl. Phys. 2010, 108, 123518. (4) Matsuzawa, T.; Aoki, Y.; Takeuchi, N.; Murayama, Y. J. Electrochem. Soc. 1996, 143, 2670−2673. (5) Zhong, R. X.; Zhang, J. H.; Zhang, X.; Lu, S. Z.; Wang, X. J. Appl. Phys. Lett. 2006, 88, 201916. (6) Liu, Y. L.; Lei, B. F.; Shi, C. S. Chem. Mater. 2005, 17, 2108− 2113. (7) Yamamoto, H.; Matsuzawa, T. J. Lumin. 1997, 72−74, 287−289. (8) Lei, B. F.; Liu, Y. L.; Zhang, J. W.; Meng, J. X.; Man, S. Q.; Tan, S. Z. J. Alloys Compd. 2010, 495, 247−253. (9) le Masne de Chermont, Q.; Chanéac, C.; Seguin, J.; Pellé, F.; Maîtrejean, S.; Jolivet, J. P.; Gourier, D.; Bessodes, M.; Scherman, D. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 9266−9271. (10) Jia, D.; Wu, B. Q.; Zhu, J. C.A. Patent 00100388, 2000. (11) Jia, D. D.; Zhu, J.; Wu, B. Q. J. Electrochem. Soc. 2000, 147, 386− 389. (12) Diallo, P. T.; Boutinaud, P.; Mahiou, R.; Cousseins, J. C. Phys. Status Solidi A 1997, 160, 255. (13) Murazaki, Y.; Arai, K.; Ichinomiya, K. Rare Earth 2000, 36, 146. (14) Fu, J. Electrochem. Solid-State Lett. 2000, 3, 350−351. (15) Ju, Z. H.; Wei, R. P.; Zheng, J. R.; Gao, X. P.; Zhang, S. H.; Liu, W. S. Appl. Phys. Lett. 2011, 98, 121906. (16) Lei, B. F.; Yue, S.; Zhang, Y. Z.; Liu, Y. L. Chin. Phys. Lett. 2010, 27, 037201. (17) Zhang, J. C.; Hu, R.; Qin, Q. S.; Wang, D.; Liu, B. T.; Wen, Y.; Zhou, M. J.; Wang, Y. H. J. Lumin. 2012, 132, 2590−2594. (18) Pan, Z. W.; Lu, Y. Y.; Liu, F. Nat. Mater. 2012, 11, 58−63. (19) Liu, Z. Q.; Stevens-Kalceff, M.; Riesen, H. J. Phys. Chem. C 2012, 116, 8322−8331. (20) Longo, V. M.; das Graça Sampaio Costa, M.; Zirpole Simões, A.; Rosa, I. L.; Santos, C. O.; Andrés, J.; Longo, E.; Varela, J. A. Phys. Chem. Chem. Phys. 2010, 12, 7566−7579. (21) Liu, Y. L.; Kuang, J. Y.; Lei, B. F.; Shi, C. S. J. Mater. Chem. 2005, 15, 4025−4031.

Na2CaSn2Ge3O12 (sample S0) is around 3.43 eV, corresponding to the host absorption at 255 nm. We have no idea of the exact location of the Sm3+ levels relative to the substrate levels. Because we are only interested in the excitation, emission, and persistent luminescence process of Sm3+ in Na2CaSn2Ge3O12 host, the Sm3+ energy levels were placed in the valence and conduction band merely for explanatory purposes. Under the ultraviolet light irradiation, the valence band electrons are promoted to the conduction band, generating excited carriers (free electrons and holes) in the Na2CaSn2Ge3O12 matrix (process 1), and the luminescence centers Sm3+ acquired most of the excitation energy which related to the excited carries from the host through the energy transfer (process 2), and eventually generate the characteristic emissions of Sm3+ ions as the luminescence (process 5). However, some of the excited carriers were captured by different trapping centers through a relaxation process (process 3). After the ultraviolet light was turned off, the trapped carriers released by the thermal activation at room temperature were passed to Sm3+ ions through the valence band and conduction band (process 4), finally giving rise to the reddish orange light emitting LPP of Sm3+ because the energy transferred from the trap has a slow release ratio.

■

CONCLUSIONS In conclusion, a novel reddish orange LPP phosphor Sm3+doped Na2CaSn2Ge3O12 was synthesized by conventional solidstate reaction which can last for over 4.8 h after being irradiated by 254 nm UV light. The PL spectra and the afterglow decay curves revealed that the optimum composition of phosphor was Na2CaSn2Ge3O12: 1.0 mol % Sm3+. Thermoluminescence 13880

dx.doi.org/10.1021/ic401262w | Inorg. Chem. 2013, 52, 13875−13881

Inorganic Chemistry

Article

(22) Xia, Z. G.; Chen, D. M. J. Am. Ceram. Soc. 2010, 93, 1397−1401. (23) Lei, B. F.; Li, B.; Zhang, H. R.; Li, W. L. Opt. Mater. 2007, 29, 1491−1494. (24) Lei, B. F.; Man, S. Q.; Liu, Y. L.; Yue, S. Mater. Chem. Phys. 2010, 124, 912−915. (25) We cannot exactly attribute the impurities (please refer to the Supporting Information for details). The content of impurities were decreased greatly just in the absence of H3BO3 without changing the other experimental conditions, however, pure product was still not obtained. (26) Durif, A.; Maupin, G. Acta Crystallogr. 1961, 14, 440−441. (27) Li, Y. C.; Chang, Y. H.; Chang, Y. S.; Lin, Y. J.; Laing, C. H. J. Phys. Chem. C 2007, 111, 10682−10688. (28) Poulios, D. P.; Spoonhower, J. P.; Bigelow, N. P. J. Lumin. 2003, 101, 23−33. (29) Tauc, J.; Grigorovici, R.; Vancu, A. Phys. Status Solidi B 1966, 15, 627−637. (30) Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys. 1968, 49, 4412−4423. (31) Dexter, D. L.; Schulman, J. H. J. Chem. Phys. 1954, 22, 1063− 1070. (32) Blasse, G. Philips Res. Rep. 1969, 24, 131−144. (33) Im, W. B.; Fellows, N. N.; DenBaars, S. P.; Seshadri, R.; Kim, Y. I. Chem. Mater. 2009, 21, 2957−2966. (34) Luo, H. D.; Liu, J.; Zheng, X.; Han, L. X.; Ren, K. X.; Yu, X. B. J. Mater. Chem. 2012, 22, 15887−15893. (35) Hou, D. J.; Liu, C. M.; Ding, X. M.; Kuang, X. J.; Liang, H. B.; Sun, S. S.; Huang, Y.; Tao, Y. J. Mater. Chem. C 2013, 1, 493−499. (36) Lei, B. F.; Li, B.; Zhang, H. R.; Zhang, L. M.; Cong, Y.; Li, W. L. J. Electrochem. Soc. 2007, 154, H623−H630. (37) Liu, C. B.; Che, G. B.; Xu, Z. L.; Wang, Q. W. J. Alloys Compd. 2009, 474, 250−253. (38) Lei, B. F.; Machida, K.; Horikawa, T.; Hanzawa, H.; Kijima, N.; Shimomura, Y.; Yamamoto, H. J. Electrochem. Soc. 2010, 157, J196− J201. (39) McKeever, S. W. S. Thermoluminescence of Solids, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1985. (40) Trojan-Piegza, J.; Niittykoski, J.; Hölsä, J.; Zych, E. Chem. Mater. 2008, 20, 2252−2261. (41) Gao, X. P.; Zhang, Z. Y.; Wang, C.; Xu, J.; Ju, Z. H.; An, Y. Q.; Liu, W. S. J. Electrochem. Soc. 2011, 158, J405−J408. (42) Lempicki, A.; Glodo, J. Nucl. Instrum. Methods Phys. Res., Sect. A 1998, 416, 333−344. (43) Chen, R. J. Electrochem. Soc. 1969, 116, 1254−1257. (44) Emsley, J. The Elements; Clarendon Press: Oxford, U.K., 1991. (45) Clabau, F.; Rocquefelte, X.; Le Mercier, T.; Deniard, P.; Jobic, S.; Whangbo, M.-H. Chem. Mater. 2006, 18, 3212−3220. (46) Lin, Y. H.; Tang, Z. L.; Zhang, Z. T.; Nan, C. W. Appl. Phys. Lett. 2002, 81, 996. (47) Cheng, B. C.; Fang, L. T.; Zhang, Z. D.; Xiao, Y. H.; Lei, S. J. J. Phys. Chem. C 2011, 115, 1708−1731. (48) Maldiney, T.; Lecointre, A.; Viana, B.; Bessière, A.; Bessodes, M.; Gourier, D.; Richard, C.; Scherman, D. J. Am. Chem. Soc. 2011, 133, 11810−11815. (49) Qiu, J. R.; Miura, K.; Inouye, H.; Fujiwara, S.; Mitsuyu, T.; Hirao, K. J. Non-Cryst. Solids 1999, 244, 185−188. (50) Clabau, F.; Rocquefelte, X.; Jobic, S.; Deniard, P.; Whangbo, M.H.; Garcia, A.; Le Mercier, T. Chem. Mater. 2005, 17, 3904−3912. (51) Lei, B. F.; Zhang, H. R.; Mai, W. J.; Yue, S.; Liu, Y. L.; Man, S. Q. Solid State Sci. 2011, 13, 525−528.

13881

dx.doi.org/10.1021/ic401262w | Inorg. Chem. 2013, 52, 13875−13881